The invention relates to a biodegradable thermoplastic material and to a process for preparing it.
Typically, thermoplastic materials have been based on polymers that are synthetically prepared from hydrocarbon or hydrocarbon-like raw materials. They thus find their origin in the petrochemical industry.
Meanwhile, these materials have become associated with environmental problems. Their production is relatively energy intensive and produces large quantities of carbon dioxide as a side product. Carbon dioxide is often believed to cause, or at least contribute to, global warming. Furthermore, when discarded as waste, these materials degrade only very slowly, which means that they constitute a prolonged burden on the environment.
In order to alleviate these problems, various alternative materials have been developed. These alternative materials have mostly been devised with an eye to their advantageous (bio)degradation profile, when discarded after use. In the context of the invention, a biodegradable material is a material which, in a biological environment, degrades in a relatively short period of time to compounds which are preferably water soluble and non-toxic. The degradation may proceed by hydrolysis under influence of light, air, water and/or micro-organisms.
Most of the known biodegradable thermoplastic materials are either also based on hydrocarbon sources, or based on natural raw materials (monomers) or even natural polymers, such as cellulose, starch, polylactic acid, keratin, and the like. These natural raw materials are, more or less intrinsically, biodegradable. Furthermore, they have the advantage that they originate from renewable sources and will therefore always be available. Natural polymers are, however, generally not thermoplastic. In order to achieve that property, the materials are typically processed (often extruded) in combination with a plasticizer. Of course, the biodegradable properties of a suitable plasticizer are to be considered in its selection.
Unfortunately, in practice there are not many choices for the plasticizer. Usually, either water, urea, glycerol or a low aliphatic or aromatic ester is selected. Problems that are encountered are that these plasticizers either are insufficiently compatible with the biodegradable polymer, or may leach out of the product, which in its turn will become brittle and may even fall apart. This problem is particularly encountered in applications wherein the product is used in a humid or aqueous environment, i.e. when it is brought into contact with water. This disadvantage puts a serious limitation on the applications of the biodegradable thermoplastic material. It moreover means that the (mechanical) properties of the material deteriorate rather fast, making it unsuitable for use long before its biodegradation takes effect.
The present invention seeks to overcome the problems associated with the known biodegradable thermoplastic materials from natural polymers. In particular, it is an object of the invention to provide a material, which is biodegradable and has good thermoplastic and mechanical properties, which material is highly compatible with biodegradable plasticizers. It is furthermore an object of the invention that the favorable properties of the biodegradable thermoplastic material remain apparent over a prolonged period of time, preferably at least until biodegradation affects said properties.
Surprisingly, it has been found that these objects can be reached by incorporating a specific clay into a biodegradable thermoplastic material. Accordingly, the invention relates to a biodegradable, thermoplastic material comprising a natural polymer, a plasticizer and a clay having a layered structure and a cation exchange capacity of from 30 to 250 milliequivalents per 100 gram.